Process for making a stencil screen coating and composition therefor



United States Patent 3,170,791 PROCESS FOR MAKING A STENCIL SCREEN COAT-ING AND COMPOSITION THEREFOR I Clarence T. Cunningham, Alton, Ill.,assignor to Owens- Illinois Glass Company, a corporation of Ohio N0Drawing. Filed Feb. 10, 1961, Ser. No. 88,296 12 Claims. (CI. 96-35)This invention relates to a stencil screen and more particularly to acoating for use in filling the voids of a stencil screen wherebyportions of the coating are subsequently removed according tophotographic methods to form a preselected pattern of voids on thescreen. This application is a continuation-in-part of copendingapplication Serial No. 721,337, filed March 14, 1958.

In the production of stencil screens, gelatin has been Widely used as acoating therefor because of its ready availability and low cost. Inmaking such stencil screen compositions, the gelatin is first dissolvedin hot water and a light-sensitizing agent and a coloring dye are addedthereto. The purpose of the dye is to make the composition more visibleso that the ultimate user of the screen can readily perceive whichportion of the screen is coated therewith. The viscous gelatin solutionis then coated onto the fine mesh screen, either by a squeegee or byimmersing the screen while flat in a shallow pan containing some of thecomposition so that the composition will just penetrate the intersticesof the screen so as to surround them and provide a physical bond to the.screen. The coated screen is then dried, for instance, in a mild hot-airoven.

The light-sensitizing agent enables a photographic process to be used inmaking a stencil of the coated screen. A film in the nature of an opaquepositive of the desired decoration or printing is placed over the finemesh screen coated with the light-sensitized composition, and thisassembly is exposed to a light source such as an arc lamp. Thearrangement is such that the light issuing therefrom will pass throughthe negative and contact the coated screen at those areas outlined bythe transparent portion of the negative. The light contacted compositionis con verted to a state of Water insolubility, whereas the portion ofthe composition which is not contacted by light remains water soluble.

The coated screen is then given a mild wash With Water to remove theWater soluble portion, and thereby yield the final stencil which bears acoating outlining the desired decoration or printing. The portion of thescreen bearing the water-insoluble coating composition has itsinterstices filled with this composition so that the decoratingmaterial, such as printing ink, paint, vitreous enamel, glaze, or color,will not pass thereth-rough.

Stencil screens so produced are conveniently used to apply a color orprinting to a substrate such as glassware, e.g., bottles, tumblers, andthe like. The stencil held in a frame is brought into contactingrelationship with a tumbler or bottle held in a chuck. Decorating mediumor color on the screen thus is forced through the unfilled intersticesof the stencil screen by a squeegee onto the underlying surface in thedesired pattern. After either the stencil or the bottle are retracted sothat they are out of contact, the bottle or tumbler is passed into. anoven for baking, in the case of a heat-hardenable decorating material.

Heat, of course, is not always necessary, inasmuch as cold colors havebeen developed which will harden quickly under ambient conditions. Therigorous service conditions to which the stencils are exposed frequentlycause the stencil screen to crack, necessitating the shut-down of theautomatic equipment for substitution of new screens therein. Even if nodefinite cracking occurs, the stencil screens wear out due to gradualdegradation of the coating.

g In an effort to enhance the life of these screens, the art hasresorted to several modifications. In one embodiment, the screen whichhas been coated with the gelatin composition in the desired pattern anddried (as described above) is dipped into a glycerin bath. The glycerinadheres to the coated stencil screen, and the screen is then againdried. The glycerin overcoat is found to impart a slight increase instrength to the coating and, consequently, longer life of the stencil inthe automatic equipment. Another embodiment which has been utilizedinvolves the inclusion of glycerin in the melted gelatin solution.Glycerin, being water soluble, readily goes into solution with the watercontaining the melted gelatin. It has been found that this also slightlyupgrades the coating composition, thereby increasing the life of thescreen formed therefrom Unfortunately, neither of the above embodimentsis so advantageous that it really justifies the expense of themodification Thus, the coating of the stencil screen with glycerinnecessitates an extra step in the manufacture of the screens, andconsequently increases the cost Further, the cost of glycerin itselfadds to the expense of the ultimate screen.

Accordingly, it is an object of the present invention to provide animproved stencil screen masking composition which permits the productionof a stencil screen possessed of longer life than those knownheretofore.

It is another object of this invention to provide such a stencil screencoating composition which is relatively inexpensive, can be produced bya relatively simple process, and yet yields a stencil screen of improvedquality.

It is yet another object of this invention to provide a stencil screencomposition which, by reason of its greater strength and consequentlonger life, materially reduces the down time for the automaticequipment used in applying decorations to a substrate.

It is still another object of this invention to provide acompositionwhich is tougher and less apt to crack under the rigorousservice. conditions encountered with auto matic color decorating andprinting machines.

The above and many other objects, features and advantages of the presentinvention will become apparent to those skilled in the art from thefollowing detailed description and the examples set forth, in whichreference Will be had to several preferred embodiments.

Basically, the present invention is predicated upon the discovery thatan improved gelatinous stencil screen masking composition may beproduced by including in the aqueous solution of gelatin an ester ofphthalic acid such as the dialkyl ester wherein the alkyl contains from2 to 10 carbon atoms.

Example I An improved stencil screen composition of the presentinvention was prepared according to the following formulation:

The above ingredients were combined in the following manner: cc. ofwater at a temperature of. 180 F. Was poured into a glue pot. The dyeand gelatin were added, and the mixture heated to about F. The

heat was maintained until the gelatin had gone into solution (about 30minutes). Next, the dibutyl phthalate was added to the solution and themixture stirred. In a separate container, the sodium bichromate wasdissolved in 400 cc. of hot water, and this solution was poured into thegelatin solution and mixed until blended. This took about three minutes.Next, hot water was run through a colloid mill to bring the mill up tothe same temperature as the previously prepared mixture, e.g., about 175F. When the mill was thus warmed, the gelatinous mixture was run throughthe mill five times in order to more or less homogenize the ingredients.The completely mixed composition was then strained through a 165 meshstainless steel screen into a shallow pan maintained at about 140 F. Itwas allowed to set for about five minutes to allow air bubbles to riseto the surface of the liquid. The exposed surface of the liquid was thenscraped to remove the bubbles so that the screen could be floated. Tofloat a screen, it is held in a horizontal position and just immersed inthe composition. When it is lifted slowly therefrom, the bottom side ofthe screen will be formed to be more heavily coated than the top side.The composition, of course, does penetrate through the interstices ofthe screen as described hereinbefore, but, as indicated, only enough iscontained on the upper side of the screen to insure its being retainedthereby. The coated screen was then dried in a darkroom, after which anopaque positive film of the desired design, and in which the ultimatelydesired printing or lettering corresponds to the opaque part of the filmand the remaining areas thereof are translucent so as to permittransmission of light therethrough, was placed over the screen, and theassembly exposed for about one minute to light issuing from an arc lamplocated on the film side of the assembly. The portion of the compositioncontacted by the light is converted thereby to a cured state which isinsoluble in water. The screen was given a mild wash with warm water,whereupon the portion of the coated screen which had not been exposed tolight washed away, leaving the stencil screen bearing a water-insolublecoating which outlined the desired decoration, printing, or the like.This stencil screen was found to be extremely tough as compared to theconventional gelatin screens or to the gelatin screens which had beenmodified by overcoating with glycerin or combining glycerin with themelted gelatin in the formulation thereof. The stencil screen was notsubject to cracking, and was capable of undergoing a substantiallygreater number of cycles in automatic decorating equipment withoutmaterial degradation.

Example 11 An improved stencil screen coating composition of theinvention was prepared according to the same formulation and in the samemanner as described in connection with Example I, except that dioctylphthalate was substituted for dibutyl phthalate. It was found that thecomposition yielded a stencil screen which was possessed of the samedesirable qualities as the stencil produced according to Example I,e.g., longer life, tougher, non-cracking, and, at the same time,relatively inexpensive.

Example III An improved stencil screen coating composition of theinvention was prepared according to the same formulation and in the samemanner as described in connection with Example I, except that diCarbitol phthalate (the diester of phthalic acid and diethylene glycolmonoethyl ether) was substituted for dibutyl phthalate. It was foundthat the composition yielded a stencil screen which was possessed ofsimilar desirable qualities, e.g., longer life, tougher, non-cracking,and, at the same time, relatively inexpensive.

Example IV Following the aforesaid procedure, 25 grams of solid granulargelatin, technical grade, was dissolved in 100 grams of water. To thiswas added 1 gram of solid Monastral Blue dye and 4 grams of potassiumdichromate light sensitizer. This solution was combined with 25 grams ofdiethyl phthalate and blended for five minutes The resulting solutionwas applied to a stencil screen and exposed by an arc lamp. The imageproduced was clear and sharp, and the resulting stencil screen had thedesired qualities of the screens of the aforesaid examples.

Example V Again following the aforesaid procedure, 25 grams of solidgranular gelatin, technical grade, was dissolved in 100 grams of water.To this was added 1 gram of solid Monastral Blue dye and 4 grams ofpotassium dichromate light sensitizer. This solution was combined with25 grams of di-Z-ethylhexyl phthalate and blended for five minutes. Theresulting solution was applied to a stencil screen and exposed by an arclamp. The image produced was clear and sharp, and the stencil had theproperties of the aforesaid screens.

The term phthalic acid ester as used herein is intended to include anester derived from either phthalic acid or phthalic anhydride, inasmuchas the resulting ester will be essentially the same regardless of whichis used in forming it.

The amount of phthalic acid ester used in forming the improved stencilscreen coating composition of the invention is preferably about equal toor slightly in excess of the amount of gelatin on a weight basis.However, tests have shown that the gelatin may be present in amounts offrom 30 to parts by weight, based upon the combined weight of the esterand the gelatin with the ester then being present in amounts of from 70to 20 parts by weight.

Exactly why the modification of gelatin by the inclusion of the dialkylester of phthalic acid wherein the alkyl has from 2 to 10 carbon atoms,such as diethyl phthalate, dibutyl phthalate, dioctyl phthalate, etc.,and di Carbitol phthalate advantageously improves the properties in themanner described is not known. However, and although it is not intendedto be bound by any theory expressed herein, it is possible that thesematerials, although being water-insoluble (whereas glycerin iswater-soluble), contribute an internal physical plasticization, and thuslend to the final composition a resiliency which is reflected in theimproved service performance of the stencil screens.

Other light-sensitizing agents than sodium bichromate may, of course, beused in forming the masking compositions of this invention. Thus, any ofthe conventional light-sensitizing agents (essentially oxidizingmaterials) maybe used, although the dichromates of potassium, sodium,and/or ammonium are preferred.

The composition of this invention may be applied to metallic wirescreens or to silk or nylon screens with equal facility. Where appliedto wire, the composition may be removed by burning it off with flame,thus permitting the wire screen to be salvaged and reused, if desired.For nylon or silk screens, a solution of bleach such as a hypochloriteis conveniently used to remove the composition.

While there has been disclosed certain embodiments of the invention, itis not thereby desired or intended to be limited solely thereto, for theprecise proportions of the materials utilized may be varied andequivalent chemical materials may be employed, if desired, withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

Having fully described the invention, what is claimed 1. A stencilscreen comprising a screen base and a coating on said screen base, saidcoating filling the voids of the screen base and wherein subsequentlyportions of said coating are removed according to photographic methodsin order to form a preselected pattern of voids on said screen base,said coating comprising an aqueous mixture of (a) gelatin, (b) a lightsensitizing agent to permit photographic method production of thedesired pattern by rendering water insoluble that portion of saidcoating exposed to light, and (c) a dialkyl phthalate wherein the alkylgroup contains from 2 to of carbon atoms, the amount of (a) being from30 to 80 parts by weight based on the combined weight of (a) and (c).

2. A stencil screen as defined in claim 1 wherein the light sensitizingagent is a dichromate light sensitizing agent.

3. A stencil screen as defined in claim 1 wherein the amount of saiddialkyl phthalate to said gelatin is substantially equal by weight.

4. A stencil screen as defined in claim 1 wherein is (c) is diethylphthalate.

5. A stencil screen as defined in claim 1 wherein (c) is dibutylphthalate.

6. A stencil screen as defined in claim 1 wherein (c) is dioctylphthalate.

7. A stencil screen as defined in claim 1 wherein (c) is di-Z-ethylhexylphthalate.

8. A stencil screen comprising a screen base and a coating on said base,said coating filling the voids of the said screen base and whereinsubsequently portions of the coating are removed according tophotographic methods of production to form a preselected pattern on saidscreen base, said coating comprising an aqueous mixture of (a) gelatin,([2) a light sensitizing agent to permit photographic method productionof the desired pattern by rendering Water insoluble that portion of saidcoating exposed to light, and (c) the diester of phthalic acid anddiethylene glycol monoethyl ether, the amount of (a) being from 30 to 80parts by Weight based upon the combined weight of (a) and (c).

9. A stencil screen as defined in claim 8 wherein the light sensitizingagent is a dichromate light sensitizing agent.

10. A stencil screen comprising a screen base and a coating on saidbase, said coating filling the voids of the said screen base and whereinsubsequently portions of the coating are removed according tophotographic methods to form a preselected pattern of voids on saidscreen base, said coating comprising an aqueous mixture of:

G. Water 1000 Gelatin 230 Dibutyl phthal-ate 260 Dye 12 Dichromate lightsensitizing agent 20 11. In the method of forming a preselected patternof voids on a stencil screen comprising the steps of filling the voidsof said stencil screen with a coating of an aqueous mixture of gelatinand a light sensitizing agent, dry-ing said coating on said screen,placing an image containing member having transparent and nontransparentportions before said screen, exposing said screen to light passingthrough said transparent portion of said image containing member, thatportion of the gelatin coating in said voids of said screen contacted bysaid light bein rendered water insoluble, the remainder of said gelatincoating on said screen remaining water soluble, and washing said screento remove said water soluble portion of said gelatin coating, said voidscontaining said Water insoluble coating thus defining a pattern of theconfiguration of the transparent portion of said image containingmember, the improvement wherein said aqueous mixture of gelatin andlight sensitizing agent contains a dialkyl phthalate wherein the alkylgroup has from 2 to 10 carbon atoms, the amount of gelatin in saidsolution being from 30 to parts by weight based on the combined weightof gelatin and phthalate.

12. In the method of forming a preselected pattern of voids on a stencilscreen comprising the steps of filling the voids of said stencil screenwith a coating of an aqueous mixture of gelatin and a light sensitizingagent, drying said coating on said screen, placing an image containingmember having transparent and nontransparent portions before saidscreen, exposing said screen to light passing through said transparentportion of said image containing member, that portion of the gelatincoating in the said voids of said screen contacted by said light beingrendered water in soluble, the remainder of said gelatin coating on saidscreen remaining water soluble, and washing said screen to remove saidwater soluble portion of said gelatin coating, said voids containingsaid water insoluble coating thus defining a pattern of theconfiguration of the transparent portion of said image containingmember, the improvement wherein said aqueous mixture of gelatin andlight sensitizing agent contains a diester of phthalic acid anddiethylene glycol monoethyl ether, the amount of gelatin in saidsolution being from 30 to 80 parts by weight based upon the combinedweight of gelatin and diester.

References Cited by the Examiner UNITED STATES PATENTS 2,218,255 10/40Weyers et a1. 99-111 2,322,027 6/43 Jelley et al. 9697 2,685,510 8/54Yackel 96-35 X 2,801,171 7/57 Fierke et a1. 9697 2,940,854 6/60 Gray96-94 2,943,936 7/60 Spechler 9635 X 2,980,534 4/61 Printy et a l. 96-93X OTHER REFERENCES Kosloff: Screen Process Printing, Signs of the TimesPublishing Co. Cincinnati, 1950, pages 91-97.

Doolittle: The Technology of Plasticizers, John Wiley and Sons, New York1954, pages 951, 952 and 962.

NORMAN G. TORCHIN, Primary Examiner.

MILTON STERMAN, PHILIP E. MANGAN,

Examiners.

11. IN THE METHOD OF FORMING A PRESELECTED PATTERN OF VOIDS ON A STENCILSCREEN COMPRISING THE STEPS OF FILLING THE VOIDS OF SAID STENCIL SCREENWITH A COATING OF AN AQUEOUS MIXTURE OF GELATIN AND A LIGHT SENSITIZINGAGENT, DRYING SAID COATING ON SAID SCREEN, PLACING AN IMAGE CONTAININGMEMBER HAVING TRANSPARENT AND NONTRANSPARENT PORTIONS BEFORE SAIDSCREEN, EXPOSING SAID SCREEN TO LIGHT PASSING THROUGH SAID TRANSPARENTPORTION OF SAID IMAGE CONTAINING MEMBER, THAT PORTION OF THE GELATINCOATING IN SAID VOIDS OF SAID SCREEN CONTACTED BY SAID LIGHT BEINRENDERED WATER INSOLUBLE, THE REMAINDER OF SAID GELATIN COATING ON SAIDSCREEN REMAINING WATER SOLUBLE, AND WASHING SAID SCREEN TO REMOVE SAIDWATER SOLUBLE PORTION OF SAID GELATIN COATING, SAID VOIDS CONTAININGSAID WATER INSOLUBLE COATING THUS DEFINING A PATTERN OF THECONFIGURATION OF THE TRANSPARENT PORTION OF SAID IMAGE CONTAININGMEMBER, THE IMPROVEMENT WHEREIN SAID AQUEOUS MIXTURE OF GELATIN ANDLIGHT SENSITIZING AGENT CONTAINS A DIALKYL PHTHALATE WHEREIN THE ALKYLGROUP HAS FROM 2 TO 10 CARBON ATOMS, THE AMOUNT OF GELATIN IN SAIDSOLUTION BEING FROM 30 TO 80 PARTS BY WEIGHT BASED ON THE COMBINEDWEIGHT OF GELATIN AND PHTHALATE.